Intermetallic compounds are well suited for high performance and high temperature applications, such as engine applications, because of their high strength at elevated temperatures and good oxidation resistance. For example, nickel aluminides such as Ni.sub.3 Al and NiAl possess excellent oxidation resistance and strength at high temperatures. In addition, the light-weight titanium aluminide, Ti.sub.3 Al, offers attractive strength-to-weight and elastic modulus-to-weight ratios.
Despite these advantages, commercial growth of these aluminide alloys has been hampered. This is due primarily to the intrinsic brittleness of these polycrystalline intermetallic compounds. Previous research has generally focused on improving the single phase aluminide alloys and/or alloy additions to such single phase alloys, for improvement of the much-needed room temperature ductility as well as for retention (or enhancement) of high temperature strength.
An alternative approach for achieving similar improvements in physical characteristics by optimizing the characteristics of the diverse aluminides, has been to employ multiphase Ni-Al-Ti systems which contain various combinations of the Ni.sub.2 AlTi, Ni.sub.3 (Al,Ti) and Ni(AlTi) phases. Multiphase Ni-Al-Ti alloys which were induction melted, and then cast, have shown excellent strength in compression over the temperature range from room temperature up to about 1000.degree. C., with corresponding room temperature compressive ductility of about 0.4% to about 15%. This is a substantial improvement over the single-alloy aluminide systems. Within these multiphase Ni-Al-Ti systems, it is believed that optimum properties were obtained with alloys consisting of the Ni.sub.2 AlTi and Ni.sub.3 (Al,Ti) phases, with the room temperature ductility being directly related to the amount of Ni.sub.3 (Al,Ti).
However these cast alloys did not have a uniform phase distribution nor uniform grain size, even after 50 hours of homogenization at about 1150.degree. C., indicating widespread chemical segregation within the alloy. These are typical problems associated with castings although they appear to be more severe for these multiphase Ni-Al-Ti intermetallic aluminide alloys. Therefore, it would be desirable to provide a method for forming these multiphase Ni-Al-Ti alloys which does not employ traditional casting techniques.
Powder metallurgy processing has the potential to eliminate the disadvantages inherent in the Ni-Al-Ti castings since this technique generally produces an alloy with uniform composition and phase distribution, as well as fine grain size, thereby improving the ductility of the alloy at room temperature. For these reasons, powder metal processing, which utilizes hot compaction methods, i.e., hot extrusion or hot isostatic pressing (HIPing) of atomized prealloyed powders, has been the primary method to produce single phase intermetallic aluminide alloys.
Nevertheless, there has been limited success with the multiphase alloys. Also, these conventional powder metallurgy processes are characterized by being relatively costly. In particular, the atomized Ni-Al-Ti powders are expensive. Further, the hot consolidation processes are time consuming and require expensive canning and decanning steps, yet have been necessary to achieve sufficient green strength and density within the compact because of the brittle nature of aluminide alloy powders, which prevents the use of conventional compaction techniques. (While acceptable compaction can be accomplished using plasticizers added to the powder, the properties of alloys after sintering are usually much lower with this procedure.) Lastly, any parts with complicated geometry still require costly machining steps since aluminide alloys are relatively difficult to machine except for by grinding.
Therefore what is needed is a method for producing these multiphase Ni-Al-Ti aluminide alloys which preferably employs powder metallurgy processing so as to obtain the benefits of this technology, but which does not require the use of atomized powders or hot isostatic consolidation processes. It would be even more desirable if the resulting article consisting of the Ni2AlTi and/or Ni(Al,Ti) and/or Ni3(Al,Ti) phases, formed from such a method, were near-net shape and characterized by a near-theoretical density, so as to minimize the subsequent machining of these hard materials.